1. Field of the Invention
The present invention relates to a transmission ratio varying mechanism used in a steering system of a vehicle, and more particularly to a transmission ratio varying mechanism that can vary transmission characteristics between the steering angle of a steering handle that a driver operates and the steering angle of steered wheels of the vehicle.
2. Description of the Related Art
Conventionally, a steering system of a vehicle is configured such that a transmission ratio varying mechanism is installed between a steering handle that a driver operates and steered wheels of the vehicle and so as to vary, in response to the running state of the vehicle, the transmission characteristics of the steering angle of the steered wheels with respect to the steering angle of the steering handle that the driver operates. The transmission characteristics of the steering angle of the steered wheels with respect to the steering angle of the steering handle are commonly changed by varying the transmission ratio of rotation between a steering shaft that rotates integrally with the steering handle and a steering shaft that rotates integrally with a member that steers a steering wheel. As a transmission ratio varying mechanism used in such a steering system, a transmission ratio varying mechanism using a wave gear mechanism in a mechanism portion that enables the transmission ratio of rotation to be varied has been proposed (e.g., see JP-A-2006-44402). The transmission ratio varying mechanism disclosed in JP-A-2006-44402 is configured by a stator gear connected to an input shaft, a driven gear connected to an output shaft, an elliptical wave generator connected to a motor for varying the transmission ratio, and a flexible gear that is disposed around the wave generator and simultaneously meshes with the aforementioned two gears.
In the transmission ratio varying mechanism disclosed in JP-A-2006-44402, the stator gear rotates by the rotation of the input shaft, and the flexible gear rotates while elastically deforming about the elliptical wave generator. Because the flexible gear meshes also with the driven gear, the rotation of the flexible gear is transmitted to the output shaft connected to the driven gear. When the wave generator is rotated by the motor, the meshed places between the flexible gear and the stator gear and the driven gear also synchronously rotate. Because the numbers of teeth that the stator gear and the driven gear have are different, rotation corresponding to this tooth number difference and the rotation angle of the wave generator are superposed on the output shaft.
In the case of the conventional transmission ratio varying mechanism using a wave gear in this manner, in terms of the structure of the wave gear mechanism there have been problems in that, although the input shaft is disposed so as to confine the driven gear from the outer periphery of the stator gear, it is necessary for the output shaft from the driven gear confined by the input shaft to be removed to the outside through the center of a through hole disposed in the wave generator, the structure is complicated, and therefore assembly is difficult. Further, because the flexible gear rotates while deforming due to the rotation of the input shaft, there has been the problem that a burden is applied to the input shaft. Moreover, there have also been such problems as, in terms of the principle of the wave gear mechanism, it is necessary to rotate the wave gear mechanism at a high speed because the reduction ratio to the superimposed rotation onto the output shaft with respect to the rotation of the wave generator becomes larger.